The present invention relates generally to exhaust systems for motor vehicles and, more specifically, to an on-board reductant delivery assembly for an exhaust aftertreatment system of a motor vehicle.
Exhaust aftertreatment systems for motor vehicles are known. Typically, the exhaust aftertreatment system includes an exhaust manifold connected to the engine of a motor vehicle and a close-coupled catalyst connected via an exhaust pipe to the exhaust manifold. The exhaust system also includes an underbody catalyst connected via the exhaust pipe to the close-coupled catalyst and a tail pipe connected to the underbody catalyst.
Lean NOx catalysts (LNC) and selective catalytic reduction catalysts (SCR) are known to reduce NOx emissions from diesel and lean-burn gasoline engines. In order to promote NOx reduction under oxidizing environments, certain reductants such as hydrocarbon (HC) fuels and/or urea solution or other ammonia (NH3) containing compounds must be added. To function effectively, the conditions for adding these reducing compounds to the exhaust aftertreatment system are critical. Specifically, the injected quantity must precisely match the NOx concentration in order to obtain maximum conversion and to avoid HC and/or NH3 slippage.
The amount of added catalyst also depends upon factors such as the exhaust flow rate and catalyst temperature. Therefore, a fluid metering device is needed to regulate the reductant flow. In addition, the reductant must be dispersed and mixed well with the exhaust gas to obtain optimal results of NOx reduction. The above factors require that the reductant be well atomized of vaporized before reaching the catalyst.
Prior to the present invention, one method of reductant delivery was to have a fluid metering device 9 (FIGS. 1 and 2), such as an electronic fuel injector or a metering pump to deliver a controlled quality reductant. The fluid metering device 9 has a suction line 11 connected with a reservoir 15 of reductant. The fluid metering device 9 introduces the reductant into a mixing chamber 19 provided by an enclosure or tubular fixture 23. An air compressor 25 is a source of pressurized air. A line 27 is connected between the air compressor 25 and fixture 23 to allow pressurized air to be delivered to the mixing chamber 19. The air/reductant mixture is then directed through a transfer tube 31 to a nozzle 35. The air/reductant mixture is then sprayed into the exhaust line 37 of the vehicle. Due to the high pressure differential and the orifice structure of the nozzle 35, the air reductant mixture coming out of the nozzle 35 is quickly converted into very fine droplets which quickly vaporize and mix with the exhaust gas prior to the downstream emission catalysts.
Prior to the present invention, the fluid metering device 9 and the tubular fixture 23 of the mixing chamber 19 were two separate components. The metering device 9 and the tubular fixture 23 had to be assembled closely connected in a way that the reductant could be delivered to the mixing chamber 19 without leakage or separation. The assembly of the fluid metering device 9 into the tubular fixture 23 not only required on line final assembly production time but also required additional parts such as O-rings, tubing, connectors and brackets to hold the fluid metering device 9 and tubular fixture 23 to prevent separation.
It is desirable to provide a reductant delivery assembly which couples the fluid metering device with an enclosure, which has a control volume wherein air can mix with the reductant. Placing the fluid metering device within an enclosure would lower the potential for leakage and eliminate the extra parts, time and labor required for production line assembly of two separate components. It is also desirable to provide a reductant delivery assembly wherein the weight and volume of the reductant delivery system is reduced. It is additionally desirable to provide a reductant delivery assembly which can reduce costs. It is also desirable to provide a reductant delivery assembly with an improved response time. It is also desirable to provide a reductant delivery assembly wherein the fluid delivery system is protected from overheating and wherein there is improved reductant vaporization. It is furthermore desirable to provide a reductant delivery assembly wherein the reductant is heated during the initial stages of vehicle operation after the vehicle has been stored in cold inclement weather.
To make manifest the above noted desires, the revelation of the present invention is brought forth. In one preferred embodiment of the present invention, an on-board reductant delivery assembly is provided which includes a nozzle for atomizing delivery of a reductant into the exhaust line of the vehicle. A transfer tube is connected with the nozzle for delivering the reductant. A housing having an outlet fluidly connected with the transfer tube opposite the nozzle is provided. The housing has a front end forming a mixing chamber. A rear end of the housing has first and second chambers. The first and second chambers of the housing are fluidly connected by a plurality of inlets. The inlets between the first and second chambers of the housing have fins or buffers to induce turbulent airflow within the second chamber. A pressurized air inlet is connected with the first chamber of the housing. A fluid metering device is positioned within the housing. The fluid metering device delivers a measured portion of reductant into the mixing chamber of the housing as required. The fluid metering device is electrically powered and has coils which are positioned within the second chamber of the housing. The coils have an exposed outer boundary and are cooled by the turbulent airflow which passes through the second chamber of the housing. The metering device has an outlet which is fluidly connected with the mixing chamber and a venturi throat.
It is an object of the present invention to provide an on-board reductant delivery assembly wherein the metering device is positioned within a housing which also forms an enclosure providing a mixing chamber for pressurized air and the reductant.
The above-noted object and other advantages of the present invention will become more apparent to those skilled in the art as the invention is further explained in the accompanying drawings and detailed description.